MARCH 2000
VOLUME 20
ISSUE 3
In This Issue
Managing Agricultural Phosphorus To Protect Water Quality
Phosphorus in runoff impairs surface water quality.
Concern about agricultural nutrients and water quality is nothing new in Colorado, but in the past our attention was focused mainly on impacts from nitrogen. Phosphorus (P) is now receiving much attention nationwide as an important surface water pollutant. Surface water that receives phosphorus due to soil erosion, runoff from feedlots, or irrigation suffers from the process of accelerated eutrophication. Eutrophication is the natural aging of lakes or streams brought on by nutrient enrichment. Eutrophication has been identified as the main cause of impaired surface water quality across the country. This decline in water quality restricts fishing, recreation, and drinking uses due to the increased growth of undesirable algae and aquatic weeds and to oxygen shortages caused by their death and decomposition.
Agriculture is not the only source of P in the aquatic environment. For example, the USGS estimates that almost 2 million pounds of P are added to the South Platte each year from municipal waste discharges directly into the river. They report that manure and fertilizers applied to cropland and lawns make up the majority of the P load in the basin.
One of the difficulties in achieving better management of P fertilizer is that small, economically insignificant amounts of P are enough to cause water quality impairment. Lake water concentrations of P above 0.02 ppm generally accelerate eutrophication. These values are an order of magnitude lower than P concentrations in soil solution critical for plant growth (0.2 to 0.3 ppm), emphasizing the disparity between critical lake and soil P concentrations.
Continual long-term application of fertilizer or manure at levels exceeding crop needs will increase soil P levels. In many areas with confined animal production, manures are normally applied at rates designed to meet crop N requirements but to avoid ground water quality problems created by leaching of excess N. Nitrogen-based management has been advocated by Extension and other crop advisers for many years. The result is a buildup of soil P to excessive levels over time. In many cases we now will need to recommend P based management, significantly increasing the number of acres needed to accommodate all of the manure produced. Livestock and crop producers are going to need help in understanding why they should consider implementing such a radical shift in their nutrient management approach. This newsletter attempts to provide some information to consider in managing agricultural P to protect water quality.
Table 1. Comparison of manure application rates based upon corn N versus corn P needs.
| Nutrient used to determine manure rate |
Available from Manure (1st Year)
|
Crop Nutrient Requirement**
|
Manure Rate
|
Supplemental Fertilizer Required
|
|
|
|
|
|
|
|
Nitrogen
|
|
|
|
|
|
Phosphorus (P2O5)
|
|
|
|
|
*Example based upon actual soil and manure analysis, 35-ton
silage corn yield goal for a Weld County field.
**Nitrogen requirement after residual soil nitrate and organic matter
credits. Phosphorus requirement based upon crop removal and 80%
first year availability.
